Make a Cold Frame for Herbs

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On warm days, prop open your cold frame to allow ventilation and invite pollinators in.
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A low plastic tunnel can protect seedlings. This 12-foot Flip-Top Cold Frame by FarmTek is $539.
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On warm days, prop open your cold frame to allow ventilation and invite pollinators in.
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Corrugated fiberglass can make a suitable topping for your cold frame.
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Hay bales topped by old storm windows work as cold frames made from what is on hand.

Gardening guru Eliot Coleman asserts that “the basic cold frame is the most dependable, least exploited aid for the four-season harvest.” I couldn’t agree more, whether you’re stretching the season for spinach, growing biennial flowers and herbs, or using a cold frame (also known as a winter frame) as a holding place for divisions taken from chives, sage and other cold-hardy herbs. In the fall, I often lift seedlings of catnip, lemon balm or feverfew as I find them, and slip them into a bed I can cover with a frame or low tunnel.

• DIY: Milk Jug Seed Starter

You can also use a winter frame as temporary winter quarters for overwintering plants that suffer under heavy snow, such as lamb’s ears, or for small potted herbs you plan to share. Winter frames have so many uses that eventually you will want several of them.

Your cold frame can be as unique as your garden. If you already have raised beds, you can usually find a simple way to top one or two of them with a winter cold frame. The first version I ever used was rustic, to say the least: I attached pipe strips to the outside of a raised bed and used green branches cut from the woods for hoops. The bed-turned-winter-tunnel was covered with plastic and anchored with pieces of firewood. It was such a resounding success that my winter frames have multiplied like lemon balm, incorporating ideas I’ve picked up from other year-round gardeners.

You can build a bed topper from scratch, but it’s usually simpler to reuse storm windows or shower doors found at resale shops like those run by Habitat for Humanity. These three movable cold frame setups have won thousands of fans in a wide range of climates:

Storm windows of recent manufacture (less than 30 years old, thus unlikely to be contaminated with lead), with their screens intact, are a top choice for covering winter cold frames. In spring, when you no longer need the protection of glass, you can pop in the screens to create a lightly shaded spot, protected from hail, for hardening off seedlings or propagating divisions or rooted cuttings.

Shower doors with their hinges, frame and safety handles still attached. The thick, tempered glass of shower doors can stand up to heavy loads of ice and snow (or kids climbing on them). Shower door frame toppers are not easily blown about in the wind because of their weight. I have used a humble box built of 2-by-4s topped with an old shower door for five years, and I never run out of uses for it.

Low plastic tunnels snugged in tight. My mountainside garden gets whipped by winter winds that will tear apart any plastic-covered setup except a low, tight tunnel, about 18 inches tall at its highest point. My favorite support is flexible wire fencing, bent into an arch that is then covered with plastic sheeting. Wire hoops will work, too, but the main thing is to keep the tunnel low and secure the edges well. I often bury the edges on two sides, and use boards, bricks and other weights on the other two sides. There is little need to open a low tunnel once winter gets underway, but having an easy-open edge can be important when you want to add or subtract plants from your tunnel.

Even More Ideas

Want more ideas? You can make a winter cold frame from a hay bale enclosure covered with plastic, or you might build one with bricks or concrete blocks and top it with translucent corrugated fiberglass. Your frames need not be all alike.

I like frames I can move around by myself, so size and weight are important considerations. Cold frames in very chilly climates often call for more insulation. If you live north of Zone 6, you may want to insulate the sides with a berm of soil or mulch. Especially if you live in a cloudy area, don’t neglect the inside of your cold frame. In climates with chronic winter cloud cover, you can paint the interior walls of your frames bright white or cover them with heavy-duty aluminum foil to reflect more light.

Working with Winter Frames

Winter frames shelter plants from ice, snow and treacherous winter winds, and warm the soil whenever the sun shines. They also retain that heat in the soil once the sun goes down. To maximize solar gain, locate frames so they face the winter sun (south/southwest). You can cut slanted sides, or mound soil to make the back edge of a plain box frame sit slightly higher than the front.

Scrap wood or untreated 2-by-4 or 2-by-6 pine boards are fine as framing materials, or you can upgrade to rot-resistant cedar, redwood, or locust- or termite-proof composite plastic lumber (sold as decking planks). Other options include logs, baled hay or straw, bricks, or concrete blocks. Tempered glass shower and patio doors come in all sorts of weird sizes, so it’s best to secure a top first, and then tailor the frame to match its dimensions.

When building a wood box to support a frame, use steel corner brackets to secure corners. Brackets come in different forms–some for inside the box and some for outside. The simplest (and cheapest) ones screw into the top of a frame that’s already been banged together with 3-inch box nails.

The soil inside a frame stays warmer than open ground in winter, but unventilated frames can quickly overheat during periods of mild, sunny weather. When in doubt, it is always better to vent than to risk frying your plants. If blustery winds threaten to sabotage your cold frame, place a heavy board over the box, between the frame and the top, to keep it from slamming shut. Or use hooks and eyes to fasten the open top to posts sunk into the ground alongside the frame.

When I can’t be around to monitor ventilation and bright winter sun is expected, I cover my frames with old blankets to block out light. The plants inside are better off resting in the dark than cooking in the sun, at least until winter turns to spring. I also use thick blankets, quilts or bedspreads to bump up the protection provided by my frames during severe winter storms. Snow makes a great insulating cover for winter frames, too.

Plant in Cold Frames

VEGGIES. Mache, spinach and hardy lettuces easily survive winter in frames. You can grow your own arugula, beet and carrot seed by bringing pairs of plants through winter in frames, and
allowing them to bloom and set seed in spring.

HARDY ANNUALS AND BIENNIALS. Corn-flowers, feverfew, foxglove, breadseed poppies, Johnny jump-ups, lunaria (money plant) and parsley thrive beneath cold frames. Biennial flowers fill the bloom gap between spring-flowering bulbs and summer annuals.

PERENNIALS IN TRANSITION. When planted in pots that are then sunk into the soil up to their rims, air-layered divisions taken from sage or thyme are right at home in winter frames, as are clumps of chives or garlic chives you might want to coax to life in a sunny windowsill in late winter.

Barbara Pleasant has been researching and writing about organic gardening for more than 25 years. Purchase her book Starter Vegetable Gardens (Storey Publishing, 2010).

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